8v71 fuel pump

[4shore]

Hey guys I am looking for any ideas on how to diagnose a fuel pump. I went to start my bus, it started and only ran a few seconds. Bus does have fuel in it, flap is open on blower, fuel filters have fuel in them. I did have to replace fuel line from secondary fuel filter going to left bank on engine, there was no fuel in this line. Any suggestions appreciated. Thank You Kevin

8v71
1978 MCI 8

[luvrbus]

reprime the system and you will be fine the fuel pumps don't fail often you used what fuel was in the fuel galley and it is now starting to pick it up.My bus will do that every time I change the filters so I run the primer pump for few minutes and it goes then     good luck

[4shore]

what is the best way to reprime system. also can the engine lose prime from setting. I had not started the bus for about 2 mths. I did nothing until bus would not run, then I went through the above steps

[luvrbus]

Kevin, in the search part of this board there are several topic on different ways to prime a DD some use garden sprayers.I have priming pump on mine that is the best 70 bucks you can spend for priming.They sell a electric in line pumps which are called full flow at Auto Zone install it after the primary filter on the line and install a off and on switch.With this type pump there is no need for all the plumbing some do with the bypass valves.Back to priming you need to remove the plug in top of the filter to prime it with a little pressure . And they will lose the prime if the check valve is bad ,the other guys will tell you what system works best for them     good luck

[gus]

As Luvrbus says, if the fuel supply line check valve fails the fuel drains back into the tank and you lose prime. This happened to my 671.

I assume you have a mechanical suction fuel pump at the engine. If you happen to have an electric pump at the tank all you have to do is turn it on and pump. These mechanical pumps almost never fail.

There is also a cute little thing on most buses that will shut down the engine once air pressure builds up. This is a switch for mechanics at the rear for shutting off the engine during maintenance. Since the engine shutoff needs air pressure to work the engine will run fine until the air builds up enough to shut off the engine, very frustrating when you don't know about it. Don't ask me why I am such an expert on this switch!!

[4shore]

how do you replace check valve

[Don4107]

I would not start changing parts until you find out what is wrong.  If the check valve is leaking back it will not prevent the engine from running.  Get the system primed and see if it will run.  If it runs right but looses prime after sitting, then think about the check valve. 

When working on the fuel lines you need to understand what it takes to make a good leak free connection.  Otherwise you may introduce more problems which can become difficult to troubleshoot.

I like the inline electric fuel pump.  If it is not a pump that is designed to allow fuel to flow when turned off, you must provide a path around the pump or leave it turned on when ever the engine runs.  You will use it every time you change filters or open the fuel system.  It can be mounted near the tank or engine.  Mine is in the engine compartment with the switch within reach of the filters. Very handy.  Change the filters, hit the switch, bleed the air, fire her up. 

Good luck
Don 4107

[luvrbus]

The rotary electric in line pump will flow fuel through it when turned off the piston type won't.I have had mine for 15 years I prime the system turn it off and go no bypass valves    good luck

[TomC]

I watched Don Fairchild re prime my engine. First off, if you are going to use an electric primer pump, only use it on the secondary pump AFTER the fuel pump.  I know this doesn't sound right, but doesn't take much to blow out the seals on the fuel pump.  What you want to do is to make sure the two fuel filters are full, then disconnect the fuel lines at the fuel filters and pour fuel into them filling them up.  With the fuel pump on then start the engine and let it run on the electric fuel pump for a couple of minutes, then turn it off.  If it stalls out, repeat on the fuel filter fill up and filling the fuel lines. Usually after about three tries, should have your prime back.  Also, in the future, when changing the fuel filters, first warm up the engine. Then with the engine off change the filters, refilling the new ones before reinstalling them.  When you start the engine after changing the filters, immediately rev the engine up and it will quickly pick up the prime again without stalling. Good Luck, TomC

[Don4107]

How would a pump that puts out 7-9PSI damage the seals on a pump that operates north of 50PSI?  What am I missing?



Don 4107

[pvcces]

Don, the inlet side of the pump has a vacuum when running; if the seal is exposed to inlet pressure, it may not be designed to tolerate the delivery pressure.

We see this kind of problem occaisionally with oil burner pumps, which are normally set at 100 PSI. And when they are running, they will sometimes pump air into the output.

For what it's worth.

Tom Caffrey

[4shore]

thanks for all the info, I did not change the filters, I think it must have lost prime setting, is that possible

[luvrbus]

4shore, when any work is done on the fuel supply 95 % of the time you will lose the prime you changing the hose was more than likely what caused your problem    good luck

[Don4107]

DA book shows the fuel pump drive shaft has double seals.  One to keep the fuel in the pump, one to keep the oil in the engine with a seep hole between that returns to the suction side of the pump.  So the seal sees suction for a few degrees and pressure on the rest of the circumference.   If priming with my electric pump causes the gear driven pump seals to leak, I think I needed new seals anyway.

So, how many people use a priming pump before the primary and how many use a pump after the engine driven pump?  If you use a priming pump before the engine driven pump have you had a engine driven pump seal failure?

Don 4107

[luvrbus]

Don,mine is installed after the primary just before it goes to the gear driven engine pump.Van has his before the primary and it has never been a problem for him.     good luck